Several types of color displays are known for use in mobile devices. These known devices have limitations however, including high power consumption requirements and limited color saturation capabilities. Limited color saturation refers to situations in which the display cannot distinctly display subtle color changes. An example of such a known display is an Organic Light-Emitting Diode (OLED) display. A single pixel 10 of an OLED is shown in FIG. 1. Each pixel of an OLED has a set of three color emitters 12: red 12a, green 12b, and blue 12c. Colors other than red, blue and green are generated by illuminating more than one emitter at different intensities. OLED is an emissive display technology, so no backlight is required, but when the OLED is turned off the display is no longer readable. OLED displays generally demonstrate good color saturation, but they consume significant power.
Another type of known color display is a field sequential liquid crystal display (FS LCD). An illustration of an FS LCD 20 is shown in FIG. 2. FS LCD technology does not utilize OLED type color emitters or other known types of filters. An FS LCD panel utilizes a tri-color backlight 22, typically with red 24, green 26, and blue 28 colors and a light guide 30. Behind the light guide 30 is a reflector 32 and in front of the light guide 30 is a liquid crystal layer 34 between top 36 and rear 38 pieces of glass. Liquid crystal layer 34 can be, for example, a monochrome thin film transistor (TFT) display. As illustrated in FIG. 3, in an FS LCD, the tri-color backlight 22 turns on and off individual colors one by one at a rate higher than the human eye can differentiate so that the viewer perceives a composite color made of the individual colors lit during a cycle. As shown in FIG. 3, different fields of the liquid crystal layer 34 can be set to pass light as the individual backlights are illuminated. FIG. 3 shows red 40, blue 42, and green 44 fields being sequentially formed as the respective backlight is illuminated to form a composite image 46. A wide array of colors can be created with this technique.
The rate of the sequence and the time that each backlight is illuminated is a function of, and limited by, the response time of the liquid crystal layer 34. A sixty (60) Hertz frame rate is achieved in the example shown in FIG. 3 by tripling the frame rate of the liquid crystal to 180 Hertz and displaying each color for one-third of the time or 60 or 180 cycles in a second. By this method the human eye perceives a composite image 46 as shown in the center of FIG. 3. If the response time of a liquid crystal is slowed, then eventually the user will be able to see the sequence of the backlight colors. When the rate is slow enough for the user to perceive the sequence of backlights, the user will have difficulty perceiving composite colors and will most likely see fragments of color. Color fragmentation also occurs or becomes more severe when the user either moves with respect to the display or experiences certain vibrations, such as on a bumpy car or train ride. Any degree of color fragmentation makes it difficult for the user to perceive the data being displayed, as individual images or characters may appear blurred. An ideal liquid crystal layer 34 for an FS LCD 100 would have a response time fast enough that users would not see the individual sequencing of the primary colors.
When color fragmentation becomes a problem for the user, one solution is to turn off the multi-color backlight 22, and use the FS LCD 20 as a black on “white” display. The “white” background in this mode is created by ambient light being reflected off the reflector 32 located at the back of the display. In this mode of operation, however, the black characters created by the liquid crystal have shadows caused by reflections of the characters off the reflector 32. Due to shadows and the passive nature of reflected ambient light this mode also has a low contrast ratio.